Method of freezing solution droplets and the like using immiscible refrigerants of differing densities

ABSTRACT

This disclosure describes a system for the continuous formation of frozen droplets of an aqueous salt solution. The refrigerant consists of two or more immiscible liquids of substantially different densities. Liquid droplets are injected into the lower, denser medium in a region maintained slightly above the liquid&#39;&#39;s freezing temperature. Heat is added to the lower medium and extracted from the upper medium at controlled rates. The droplets rise through the negative temperature gradient thus created in the lower medium, and through the turbulent interface between refrigerants. The frozen droplets are extracted from the surface of the upper medium.

ilnited States Patent Dunn et a]. [451 Apr. 4, 1972 54] METHOD OFFREEZING SOLUTION 3,484,946 12/1969 Sauer ..34/s DROPLETS AND THE LIKEUSING 1,393,383 11/1921 Linebarger ..264/l3 g gl g gggggg OF FOREIGNPATENTS 0R APPLICATIONS 932,997 7/1963 Great Britain ..264/l3 Inventors:Eugene Blair Dunn, South Plainfield; Gerald James Masavage, Manville,both of N.J.; Harold Alfred Sauer, l-latboro, Pa.

Appl. No.: 15,001

US. Cl ..62/74, 264/ l 3 Int. Cl. ..F25c 1/00, B29b 3/02 Field of Search..62/74, 58, 347; 34/5; 264/28, 264/13, 5

References Cited UNITED STATES PATENTS FR POWER SUPPLY HEATING CO|L,|2

Primary ExaminerWilliam E. Wayner Attorney-R. J. Guenther and Edwin B.Cave [5 7] ABSTRACT This disclosure describes a system for thecontinuous formation of frozen droplets of an aqueous salt solution. Therefrigerant consists of two or more immiscible liquids of substantiallydifferent densities. Liquid droplets are injected into the lower, densermedium in a region maintained slightly above the liquids freezingtemperature. Heat is added to the lower medium and extracted from theupper medium at controlled rates. The droplets rise through the negativetemperature gradient thus created in the lower medium, and through theturbulent interface between refrigerants. The frozen droplets areextracted from the surface of the upper medium.

12 Claims, 2 Drawing Figures -MEAN POSITION OF LIQUID INTERFACE 4FROMVESSEL CONTAINING SALT SOLUTION PATENTEDAFR 41972 3 653,222

SHEEI 1 [IF 2 IIIII N GAS OUT POSITION OF 00 LIQUID INTERFACE FROM POWERSUPPLY HEATING COIL,I2

FROM VESSEL CONTAINING SALT SOLUTION E. B. DUNN lNl/ENTORS G. J MASAVAGEH. A, SAUER ATTORNEY PATENTEDAPR M972 3.653.222

SHEET 2 OF 2 FIG. 2

METHOD OF FREEZING SOLUTION DROPLETS AND THE LIKE USING IMMISCIBLEREFRIGERANTS OF DIFFERING DENSITIES FIELD OF THE INVENTION Thisinvention relates to cryogenic systems, and particularly concerns amethod for freezing salt solution droplets rapidly, continuously, and insubstantially uniform sizes.

BACKGROUND OF THE INVENTION In the H. A. Sauer, US Pat. No. 3,484,946,there is described a method for freezing droplets in which an aqueoussalt solution is introduced into the lowermost region of a denser,immiscible refrigerant. The temperature at the injector nozzle is keptsomewhat higher than the solutions freezing point. Slightly above theinjector an abrupt temperature decrease is effected to well below thefreezing point.

In maintaining the requisite temperature gradient, however, certainproblems were encountered. For one, the temperature differentialsproduced uninhibited convective flow patterns in the refrigerant. Thesetended to limit the gradient size and occasionally cause the temperatureat the injector to dip below the freezing point of the salt solution andfreeze shut. For another, the rate of droplet injection had to belimited so as to avoid fiuid turbulence that would further disrupt thetemperature gradient. Under these conditions droplet freezing isdelayed, or extensive droplet coalescing occurs and a poor nonuniformproduct results. Mechanical baffles and other expedients to helpmaintain the thermal gradient proved unavailing. The alternative of aninordinately tall freezing column was unattractive in many respects.

Accordingly, the prime object of the invention is to freeze droplets ofa liquid dispersion such as a salt solution rapidly in a systemrequiring their injection into a lower region of a refrigerant body.

Another object of the invention is to maintain a large temperaturegradient across a relatively short column of refrigerant through whichdroplets of an immiscible liquid are rising.

A further object of the invention is to enhance the freezing capacity ofa system of the character described.

Still another object of the invention is to control and improve theuniformity of size of frozen agglomerates.

SUMMARY OF THE INVENTION The invention achieves these and other objectsby the use of a refrigerant characterized by at least two liquidrefrigerants of significantly different densities. At the injectionpoint the lower, denser refrigerant is maintained at a temperatureslightly above the freezing point of the solution. Above the injector,the denser liquid undergoes a sharp temperature drop to well below thesolution freezing point. The upper, less dense refrigerant is maintainedat a still lower and nearly uniform temperature throughout.

The interface region marks the boundary between the two refrigerants.Advantageously, the density difierential is chosen to permit control ofthe mobility of the interface. Specifically, momentum transfer to theinterface as a result of a convective process in the respectiverefrigerants, initiates and maintains a desired degree of interfaceturbulence.

The resulting enhanced heat exchange between the two refrigerantsrenders it relatively easy to establish a temperature near the freezingpoint of the solution at the injection orifice, and a sharp negativegradient just above it. Further, interfacial turbulence preventsformation of a viscous, hardtopenetrate liquid layer in the region ofthe interface.

A prime feature of the invention, accordingly, is a refrigerantconsisting of two immiscible liquids of different densities, throughwhich solution droplets rise in consequence of the buoyant forces of theliquids.

A further feature of the invention is the use of a density differentialbetween the two refrigerants that produces turbulence at the interfacewhen a temperature differential between them is established.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic side perspective viewof apparatus for practicing the inventive method; and

FIG. 2 is a schematic diagram of the refrigerant layers, showingtemperature gradients realized.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT In the specificillustrations shortly to be described, the salt in solution was aluminumsulfate. However, it should be understood that, in accordance with thedefinition of a salt, any substance which yields ions (other thanhydrogen or hydroxyl ions) in water can be processed pursuant to theInvention. More generally, solutions in which dissociation of the salt(the solute) into ions takes place may be processed. Solutions in whichionization is absent, or almost absent, also may be processed pursuantto the invention. An example of the latter is household sugar dissolvedin water.

It should be understood also that certain fluid colloidal systems, orsols, can be processed as taught herein. Older ceramics, for example,contain silica the salts of which are not soluble in water. Silicacolloidal suspensions are available, however, such as that known by thetrade name Ludox" manufactured by E. I. du Pont de Nemours and Company,Inc. In freezing colloidal systems, pursuant to the invention, it isoccasionally desirable to first mix the suspension with a binder such asa salt solution or an organic binder such as methyl cellulose.

Broadly, then, the invention embraces the freezing of liquiddispersions, including dispersions on the ionic scale of salt solutionson through fluid colloidal systems.

In a particular embodiment, appropriate materials that are soluble in asolvent for which two mutually immiscible, denser liquid refrigerantsexist, and that are also individually immiscible with the solvent, maybe processed in accordance with the general inventive teaching.

FIG. 1 depicts schematically apparatus suitable for practice of theinventive method. A cylinder 10, which, for example, may be metal orglass, is cooled in its upper portion by a system of coils 11 whichtransmit a flow of coolant such as nitrogen gas. Surrounding the lowerportion is a heat source such as the heating coil 12. Thermocouples l3,14 are placed at points A and B in the upper and lower portions,respectively. A droplet injector 15 is located at the base interior ofthe lower portion and is pressure fed with salt solution from a vessel,not shown.

Pursuant to the invention, the cylinder 10 is charged with tworefrigerants of different densities. As seen in FIG. 2, the denserrefrigerant occupies the lower or injection zone. The less denserefrigerant occupies the upper or freezing zone.

Numerous combinations of refrigerants possess the requisitecharacteristics of immiscibility both with each other and with any phaseof the salt solution, and greater density than the latter. The followingexamples are merely illustrative. The apparatus used was as depictedschematically in FIG. 2.

EXAMPLE 1 A two-liquid system consisting of Freon E5 in the injectionzone and trichloroethylene in the freezing zone was employed. Heat wasapplied to the injection zone to bring the temperature adjacent to theinjection orifice to about +6 C. By adjusting the nitrogen coolant flow,a temperature of 62 C. was maintained just across the interfacial region16. About l.5 inches above the nozzle, the temperature was 35" C. At theinterface, 2 inches above the nozzle orifice, it was 45 C. A very sharpnegative temperature gradient in the injection zone thus wasestablished, as well as very large temperature differences between theinjection orifice region and the region immediately above the interface.The overall temperature gradient realized was generally that depicted inFIG. 2.

Then, a solution of aluminum sulfate was injected using an injectionorifice diameter of 0.007 inches, at an injection rate of 7 milliletersper minute. The droplets were observed to transmit through the turbulentinterface without coalescing; and emerged at the surface of the upperrefrigerant fully frozen.

EXAMPLE 2 A two-liquid system consisting of l, 2 dichloropropane andFreon E1 was charged into the apparatus of Example 1. Adjustment of theheat input and removal as described resulted in maintaining of a sharptemperature profile characteristic of that described in Example 1.Droplet freezing was complete on reaching the upper surface. Nocoalescing of the droplets took place.

EXAMPLE 3 In the two-liquid system of Example 2 Freon E2 was substitutedfor Freon El. Again, a sharp temperature profile characteristic of thatdescribed in Example 1 was achieved; and the product results were as inExamples 1 and 2.

The term Freon relates to a product of the E. l. du Pont de Nemours andCompany, Inc, the chemical structure for which The E number in the aboveFreon designation is equal to n. Values of n from 1 to are contemplatedwithin the scope of the invention. The Freons are examples ofrefrigerants offering the normally desirable properties of nontoxicityand nonflammability.

The following applies to aqueous solutions of aluminum sulfate, and moregenerally also to many other appropriate salt solutions and solvents.

In general, the details of the profile and the magnitude of thetemperature gradient between the two zones are directly related to thedifference in densities of the two phases. The greater the densitydifferential, the greater the temperature differential. Whiletemperature differentials of 100 C. or greater between zones can beobtained and controlled, for the inventions immediate purposes, this isneither necessary nor desirable. It was found that large densitydifferentials between the liquid phases, for example, those aboveapproximately 0.5 grams/cm tend to produce a relatively stable, viscousand quiescent interfacial region, at which the flow of droplets into thefreezing zone is impeded. Moreover, the droplets in tarrying at theinterface tend to coalesce or adhere to the viscous layer, carrying thelatter with them as they rise. Process control in this case is poor.

However, a lowered density differential effects an increase in momentumtransfer to the interface by a convective process occurring in therespective zones, and stimulates interface turbulence. This chaoticmotion back and forth of molecules at the interface results in heattransfer from the lower to the upper zone. It is this latter mechanism,that might be termed the thermal valve action of the interface, thatpermits the close control of temperature gradient in the presentinvention as well as ready control of temperature at the injectionnozzle. Typically, for the region extending from the injection orificeto 2-3 inches above it, a negative temperature gradient of l520 C. perinch is adequate and readily maintainable,

Further, droplet passage from the warm zone into the cold zone issubstantially unimpeded when the viscous layer is thus avoided and theinterface turbulence is achieved. A typical range of densitydifferential suitable for the practice of the present invention is 0.30to 0.50 grams/cc.

The frozen droplet size decreases with increased injection rate.Injection rate, in turn, is determined by solution supply tank pressure.Over a pressure range of 3 to 45 psi droplet sizes varied from about 2.0mm to about 0.1 mm. For a constant supply tank pressure of 12 psi, afrozen droplet diameter of 0.3 mm 2 0.05 mm is obtained for about 90percent of the product. About percent fall on either side of theselimits.

In an alternate invention embodiment, a three-liquid refrigerant systemis used. The lowermost, densest liquid engulfs the injector in theinjection zone; the intermediate zone provides the sharp temperaturegradient; and the top zone completes the freezing. Since each interfaceacts as a thermal valve, the intermediate zone is quite easy to controlby adjustment of temperature of the two adjacent refrigerants. Anadvantage of this system is that the uppermost zone can be reduced inlength, resulting in a shorter overall unit with lower heat and heatexchange costs, and lower system thermal losses.

It is understood that the detail in which the invention has beendescribed is for the sole purpose of illustration, and is not intendedto be limiting of the invention.

What is claimed is:

1. Method of freezing liquid droplets of salt solution comprisinginjecting the droplets into the lower region of a body of immisciblerefrigerant having a density greater than the solution, characterized inthat the refrigerant consists of two immiscible liquids having asubstantial density difference, the denser medium in the region ofinjection maintained at a temperature minimally above the solutionfreezing point and the less dense medium maintained well below saidpoint said droplets after injection into the denser medium passingthrough the refrigerant of less density which is maintained below thedroplet freezing temperature to freeze the droplets while passing therethrough.

2. Method of freezing liquid droplets comprising injecting the dropletsinto a body of refrigerant, characterized in that the refrigerantcomprises two mutually immiscible refrigerants of substantiallydiffering densities, each refrigerant also being immiscible with, andmore dense than, the liquid droplets, the droplets being injected intothe denser refrigerant at a point where the temperature is slightlyabove the droplet freezing temperature said droplets after injectioninto the denser medium passing through the refrigerant of less densitywhich is maintained below the droplet freezing temperature to freeze thedroplets while passing there through.

3. Method of claim 2 further characterized in that a temperaturegradient is maintained in said body, varying from several degreescentigrade above the freezing temperature of the droplets at theinjection point to substantially below said freezing temperature at theinterface of the two refrigerants.

4. Method of claim 3 further characterized in that the densities of therefrigerants are chosen to promote interface turbulence with theapplication of a thermal gradient in said body, thus avoiding relativelyquiescent, stagnant refrigerant layers at said interface.

5. Method of claim 3, further characterized in that the liquid dropletsare a water solution of an ionizable material and one refrigerant isselected from each of groups A and B, where group A consists oftrichloroethylene and l, 2 dichloropropane, and group B consists offluorocarbons selected from Freon structures having values of n from 1to 5.

6. Method of claim 3, further characterized in that heat is applied tothe denser refrigerant and extracted from the less dense refrigerant forcontrol of said temperature gradient.

7. Method of claim 6, further characterized in that the densitydifferential between said two refrigerants is in a range of from 0.30 to0.50 grams/cc.

8. Method of claim 5 further characterized in that, for the regionextending from the injection point to 2-3 inches above same, a negativetemperature gradient in the range of l5-20 C. per inch in maintained.

9. Method of claim 3, further characterized in that liquid droplets area water solution of a nonionizable material and the refrigerants areselected, one from each of groups A and B, group A consisting oftrichloroethylene and l, 2 dichloropropane, and group B consisting offluorocarbons selected from Freon structures having values n from l to5.

10. Method of freezing liquid dispersions comprising injecting theliquid dispersion into the lower region of a body of immisciblerefrigerant having a density greater than the liquid dispersion,characterized in that the refrigerant consists of a 11. Method of claim10, characterized in that the liquid dispersion is a colloidalsuspension.

12. Method of claim 10, characterized in that said refrigerant bodycomprises three liquids with the intermediate said liquid beingmaintained in a sharp negative temperature gradient by controlledexchange of heat with the adjoining two liquids.

2. Method of freezing liquid droplets comprising injecting the droplets into a body of refrigerant, characterized in that the refrigerant comprises two mutually immiscible refrigerants of substantially differing densities, each refrigerant also being immiscible with, and more dense than, the liquid droplets, the droplets being injected into the denser refrigerant at a point where the temperature is slightly above the droplet freezing temperature said droplets after injection into the denser medium passing through the refrigerant of less density which is maintained below the droplet freezing temperature to freeze the droplets while passing there through.
 3. Method of claim 2 further characterized in that a temperature gradient is maintained in said body, varying from several degrees centigrade above the freezing temperature of the droplets at the injection point to substantially below said freezing temperature at the interface of the two refrigerants.
 4. Method of claim 3 further characterized in that the densities of the refrigerants are chosen to promote interface turbulence with the application of a thermal gradient in said body, thus avoiding relatively quiescent, stagnant refrigerant layers at said interface.
 5. Method of claim 3, further characterized in that the liquid droplets are a water solution of an ionizable material and one refrigerant is selected from each of groups A and B, where group A consists of trichloroethylene and 1, 2 dichloropropane, and group B consists of fluorocarbons selected from Freon structures having values of n from 1 to
 5. 6. Method of claim 3, further characterized in that heat is applied to the denser refrigerant and extracted from the less dense refrigerant for control of said temperature gradient.
 7. Method of claim 6, further characterized in that the density differential between said two refrigerants is in a range of from 0.30 to 0.50 grams/cc.
 8. Method of claim 5 further characterized in that, for the region extending from the injection point to 2-3 inches above same, a negative temperature gradient in the range of 15*-20* C. per inch in maintained.
 9. Method of claim 3, further characterized in that liquid droplets are a water solution of a nonionizable material and the refrigerants are selected, one from each of groups A and B, group A consisting of trichloroethylene and 1, 2 dichloropropane, and group B consisting of fluorocarbons selected from Freon structures having values n from 1 to
 5. 10. Method of freezing liquid dispersions comprising injecting the liquid dispersion into the lower region of a body of immiscible refrigerant having a density greater than the liquid dispersion, characterized in that the refrigerant consists of a plurality of immiscible liquids each substantially differing in density, the liquid refrigerant in the region of injection being maintained at a temperature minimally above the freezing point of the liquid dispersion and the less dense refrigerants being maintained well below said point said droplets after injection into the denser medium passing through the refrigerant of less density which is maintained below the droplet freezing temperature to freeze the droplets while passing there through.
 11. Method of claim 10, characterized in that the liquid dispersion is a colloidal suspension.
 12. Method of claim 10, characterized in that said refrigerant body comprises three liquids with the intermediate said liquid being maintained in a sharp negative temperature gradient by controlled exchange of heat with the adjoining two liquids. 